Silicone rubber composition

Abstract

A silicone rubber composition suitable for hot-air vulcanization under normal pressure without the disadvantages of surface tackiness and internal voids in the cured rubber products and also free from the problems of corrosiveness to metals and toxicity to workers, which comprises, as the curing agent for the organopolysiloxane, a peroxide derivative of dibenzoyl peroxide which is substituted on each of the benzene nuclei with substituted silyl, e.g. trimethyl silyl, group or a silyl-substituted methyl, e.g. trimethylsilyl methyl, group.

What is claimed is: 
     
       1. A vulcanizable silicone rubber composition which comprises: (A) 100 parts by weight of an organopolysiloxane represented by the average unit formula   R.sup.1.sub.a R.sup.2.sub.b SiO.sub.(4-a-b)/2,        in which R 1  is a monovalent hydrocarbon group free from aliphatic unsaturation, R 2  is an alkenyl group, the subscript a is a positive number in the range from 1.8 to 2.2 and the subscript b is zero or a positive number not exceeding 0.1 with the proviso that a+b is in the range from 1.8 to 2.2; and   (B) from 0.1 to 5 parts by weight of an organic peroxide represented by the general formula   X.sub.m.R.sub.n.C.sub.6 H.sub.5-m-n.CO.O.O.COC.sub.6 H..sub.5-m-n.X.sub.m.R.sub.n,        in which R is a monovalent group selected from the group consisting of methyl, ethyl, methoxy and ethoxy groups, X is a silyl group of the formula --SiR 3   3  or a silyl-substituted methyl group of the formula --CH 2  SiR 3   3 , R 3  being a hydrogen atom, a methyl group or an alkoxy group, the subscript m is 1 or 2 and the subscript n is zero, 1, or 2.   
     
     
       2. The silicone rubber composition as claimed in claim 1 wherein the organopolysiloxane as the component (A) is a diorganopolysiloxane having a linear molecular structure and each of the terminal silicon atoms has two or three vinyl groups bonded thereto. 
     
     
       3. The silicone rubber composition as claimed in claim 1 wherein the silyl group of the formula --SiR 3   3  is a trimethyl silyl group. 
     
     
       4. The silicone rubber composition as claimed in claim 3 wherein the silyl group of the formula --SiR 3   3  is bonded to the benzene nucleus at the 2- or 4-position of the nucleus relative to the carbonyl. 
     
     
       5. The silicone rubber composition as claimed in claim 1 wherein component (B) is selected from the group consisting of bis-(2-trimethylsilyl benzoyl) peroxide, bis-(4-trimethylsilyl benzoyl) peroxide and bis[2,4-di(trimethylsilyl) benzoyl] peroxide. 
     
     
       6. The silicone rubber composition as claimed in claim 1 wherein component (B) is bis-(2-methyl-4-trimethylsilyl benzoyl) peroxide or bis(2,6-dimethyl-4-trimethylsilyl benzoyl) peroxide. 
     
     
       7. The silicone rubber composition as claimed in claim 1 which further comprises at least 0.05 parts by weight of an organosilicon compound having, in a molecule, at least one hydrogen atom directly bonded to the silicon atom. 
     
     
       8. The silicone rubber composition as claimed in claim 7 which contains from 0.05 to 10 parts by weight of the organosilicon compound. 
     
     
       9. A vulcanized rubber produced by the hot air vulcanization of a silicone rubber composition of claim 1. 
     
     
       10. The vulcanized rubber of claim 9 wherein the starting organo-polysiloxane is a diorganopolysiloxane as the component (A) having a linear molecular structure and each of the terminal silicon atoms has two or three vinyl groups bonded thereto and the silyl group of the starting organic peroxide is trimethylsilyl. 
     
     
       11. The vulcanized rubber of claim 9 wherein component (B) is selected from the group consisting of bis-(2-trimethylsilyl benzoyl) peroxide, bis-(4-trimethylsilyl benzoyl) peroxide and bis-[2,4-di(trimethylsilyl) benzoyl] peroxide. 
     
     
       12. The vulcanized rubber of claim 9 wherein component (B) is bis(2-methyl-4-trimethylsilyl benzoyl) peroxide or bis(2,6-dimethyl-4-trimethylsilylbenzoyl) peroxide. 
     
     
       13. A vulcanized rubber of claim 9, further comprising from 5 to 100 parts by weight of a reinforcing filler wherein the filler is fumed silica. 
     
     
       14. A method of hot air vulcanizing a vulcanizable organopolysiloxane which comprises hot air vulcanizing the vulcanizable silicon rubber composition according to claim 1. 
     
     
       15. The method of claim 14, wherein the starting organo-polysiloxane is a diorganopolysiloxane as the component (A) having a linear molecular structure and each of the terminal silicon atoms has two or three vinyl groups bonded thereto and the silyl group of the starting organic peroxide is trimethylsilyl. 
     
     
       16. The method of claim 14, wherein component (B) is selected from the group consisting of bis-(2-trimethylsilyl benzoyl) peroxide, bis-(4-trimethylsilyl benzoyl) peroxide and bis-[2,4-di(trimethylsilyl) benzoyl] peroxide. 
     
     
       17. The method of claim 14, wherein component (B) is bis(2-methyl-4-trimethylsilyl benzoyl) peroxide or bis(2,6-dimethyl-4-trimethylsilylbenzoyl) peroxide. 
     
     
       18. The method of claim 14, the admixture further comprises from 0.05 to 10 parts by weight of an organosilicon compound having, in a molecule, at least one hydrogen atom directly bonded to the silicon atom. 
     
     
       19. A method of claim 14, the admixture further comprising 5-100 parts by weight of fumed silica.